1. Field of the Invention
This invention relates to a device for performing speed change control in a vehicular continuously variable transmission which transmits the output of the engine to the running drive wheels in a state of a continuously variable change speed, and more particularly to a device having a specific feature in the control of speed change during a braking operation.
2. Description of the Related Art
In a conventional method of performing speed change control in a vehicle on which a continuously variable transmission, a target engine speed (rpm) is mounted in correspondence to an index indicating the acceleration will of the driver (such as an engine throttle opening degree, and the amount of depression of the acceleration pedal, hereinafter, these are referred to merely as "acceleration opening degree"), and the actual engine speed is made close to the target engine speed (rpm). Japanese Patent Unexamined Publication No. Sho. 59-219557 has disclosed a speed change ratio control device in a vehicular continuously variable transmission which performs the above-described speed change control.
In the above-described device (disclosed by Japanese Patent Unexamined Publication No. Sho-59-219557), in the case where an engine brake state is established, for instance, when the vehicle runs down a slope, the speed change control is carried out in such a manner that the vehicle speed is kept constant. In this control operation, a vehicle speed which has been previously determined and stored is employed, or a vehicle speed which is obtained when a braking operation is carried out while the vehicle runs down a slope.
The engine brake running state occurs generally when the vehicle runs down a slope. The running performance required by the driver depends on the gradient of the slope. Hence, it is difficult for the above-described speed change control that the vehicle speed is kept constant to meet the requirement provided by the driver. For instance, one requirement is as follows: In the case where the gradient of a slope is low, the running is made at a relatively high speed with a small engine braking force, and in the case where the gradient of a slope is high, the running is made at a low speed with a great engine braking force. And, another requirement is as follows: In the case where the slope has many corners such as a mountain road, it is necessary to run the vehicle at a relatively low speed, and when the vehicle runs down the straight slope, it is required to run the vehicle at a relatively high speed.